The invention relates to a granule dosing system for plastics processing machines comprising: a weighing container removably retained in an operating position on a framework; a plurality of supply containers having outlets which are connected to an inlet of the weighing container via dosing mechanisms; a mixer with an inlet which is connected to an outlet of the weighing container; and a safety switch and a control device designed to switch off the dosing mechanisms and/or the mixer when the safety switch emits a switch-off signal.
Systems of this type serve for mixing plastics granules of different types in a predetermined ratio, so that the mixture can then be processed further in plastifier and, for example, an extruder connected to the downstream side of the plastifier. The dosing mechanisms are in most cases configured as gate valves. When one of the gate valves is opened, the granules from the corresponding supply container trickle into the weighing container which is supported on one or more weighing cells, so that the increase in weight of the weighing container can be monitored. When the specified amount of granules of the pertinent type has been supplied, the gate valve is closed, and suitable amounts of granules of other types are dosed-in in the same manner. When the mixer has been emptied sufficiently and is ready to receive a new batch of granules, the outlet of the weighing container is opened, so that the granules can be introduced into the mixer.
The gate valves and also the mixer have to be cleaned from time to time by the operator. Then, the open outlets of the supply containers provide access to the gate valves, and the open inlet of the mixer provides access to the interior of the mixer. However, in order to avoid a risk of injury, the dosing mechanisms and the mixer must be secured against human access while the system is running. Conventionally, this has in most cases been achieved by encapsulating the entire system in an enclosure which has a door that is secured by the safety switch.
However, the enclosure results in an increased space requirement, all the more since additional space must be provided for enabling the door to be opened. This requirement for large installation space is a particular disadvantageous because the dosing system is usually installed directly at the location of an extrusion plant which may comprise a plurality of extruders, so that the available space is limited. When, on the other hand, the dimensions of the enclosure are reduced, this limits the moving space for the operator performing the maintenance and the cleaning works.